In recent years, the spread of smartphones and tablet terminals and transmission and reception of large volume data such as moving images have been remarkably increasing data traffic of mobile communication. A promising measure for handling the increase in traffic is a base station installation method for locally installing base stations (referred to as “macro base stations” below) which cover wide ranges by using high transmission power and, in addition, multiple small cell base stations (referred to as “small cell base stations” below) which cover narrow ranges by using low transmission power, and thereby effectively increases a communication capacity. In this regard, the small cell base stations generally include pico base stations which it is assumed will be installed at hot spots at which users concentrate, and femtocell base stations which it is assumed will be installed at indoor places in which radio quality is bad. In addition, an area which enables communication with base stations is referred to as a cell.
By the way, urban areas in particular have difficulty in securing places to install base stations. Further, places at which the users concentrate also fluctuate depending on a date and a time. Under such a situation, installing multiple small cell base stations and making a small cell density higher are likely to cause a remarkable unbalance between the number of users to be accommodated and loads of small cells. As a result, there is a concern that a user throughput will be lowered in a small cell in which loads concentrate and user experience quality will be undermined.
Hence, a method for controlling coverages corresponding to small cell area sizes by adjusting transmission power of a small cell base station or directivity (tilt angle) in a vertical direction of an antenna to correct an unbalance between loads of small cells is generally employed. According to, for example, a method disclosed in International Patent Publication No. WO 2000/072618 of Patent Literature, “A METHOD FOR CELL LOAD SHARING IN A CELLULAR MOBILE RADIO COMMUNICATIONS SYSTEM”, each cell measures a traffic load of each cell (own cell), and selects a cell which largely overlaps the own cell from surrounding cells of low traffic loads when a measurement result indicates a high load. Further, an offset parameter of a signal strength which is a coverage criterion is adjusted for the selected surrounding cells and own cell to enlarge effective coverages of the selected surrounding cells in a direction of the own cell and reduce an effective coverage of the own cell in a corresponding direction.
By contrast with this, when the measurement result indicates a low load, the effective coverages of the surrounding cells are reduced, and the effective coverage of the own cell is enlarged in the corresponding direction. The above control is performed to hand over some of connected mobile stations between the cells and balance the traffic loads.
Further, it is also possible to perform coverage control by targeting at a specific mobile station. For example, a method disclosed in Japanese Unexamined Patent Application Publication No. 2002-94448 of Patent Literature, “METHOD AND APPARATUS FOR CONTROLLING DIRECTIONAL ANTENNA”, divides an area into a plurality of blocks, regards mobile stations belonging to the same block and located nearby as one group based on relative positions between a base station and mobile stations calculated based on GPS (Global Positioning System) position information, and controls a direction and a width (radiation angle) of a beam radiated from an adaptive array antenna in group units to capture one group by one beam. Such control is performed to share beams based on areas in which mobile stations locate and balance a traffic load.